The best place is the classifieds here on ducati.ms - you will need a few more posts to your name before you can place a wanted ad but if parts come up for sale you can PM the seller to seal the deal
Next place to look is ebay where just about every part is for sale. Some Countries/States/Cities have their own specific auction or classified sites that you will need to check regularly as well
A good tip is to read this forum and use the search function to get to know what parts are interchangeable between different models - sometimes the equivalent sport classic part sells for more than what the exact same part say off a monster sells for - do your homework
How do I know? I'm doing the same thing, building an '06 at the moment. So far parts have come from USA, Italy, England, Germany and New Zealand.
It is not going to be cheaper than buying a complete bike, but it is certainly a challenge and a lot fun!